Method and apparatus of determining properties of a signal transmission channel

ABSTRACT

The invention generally relates to a modem connected via a digital interface to a switched public telephone network and to a method for probing the line properties. The modem communicates with a second modem also connected via a digital interface to the same switched public telephone network. The public telephone network may incorporate voice compression devices (ADPCM G.726, G.723 etc.), digital pads (digital attenuators), robbed bit signalling and echo cancelling devices. The probing sequence of the invention uses large amplitude changes in a symbol sequence (each symbol having a duration of 125 μs). After that single amplitude change, the signal may return to the previous value or continue with the new amplitude value for a number of symbols. The number of symbols is selected to be larger than any expected impulse response of a digital impairment of the channel. The amplitude value change must be large enough to produce a sufficient result in the presence of digital pads with or without the presence of RBS.

The invention relates to a method of determining properties of a signaltransmission channel between a first subscriber end point and a secondsubscriber end point of a telephone network having a plurality ofsubscribers, wherein a first subscriber terminal is connected to saidfirst subscriber end point and a second subscriber terminal is connectedto said second subscriber end point, wherein said telephone network uponrequest of a subscriber establishes a signal transmission channelbetween said first subscriber and said second subscriber, wherein saidfirst subscriber end point is connected to the telephone network by adigital channel portion. The invention further relates to a subscriberterminal in a telephone network having a plurality of subscribers,wherein said telephone network upon request of a subscriber establishesa signal transmission channel between selected subscribers, saidsubscriber terminal being connected to a subscriber end point of saidtelephone network.

Recently, substantial progress has been made in increasing the datatransmission rates when transmitting data over conventional analoguetelephone lines. The International Telecommunications Union (ITU) haspromulgated and published various recommendations, such as V.32,V.32bis, or V.34, that are concerned with data transmission overtelephone lines. These recommendations are all based on a transmissiontechnique called quadrature amplitude modulation (QAM). QAM has provenadvantageous for the plain old telephone system (POTS) environment.

Nevertheless, the network of telephone system has undergone massivechanges in that the network is nowadays almost entirely digital. Theanalogue signals originating from a first subscriber modem are convertedat the subscriber's central office to digital representations which arecarried through the digital telephone network. At the central office ofa second subscriber, the digital signals are converted back intoanalogue signals to be driven into a second subscriber's subscriberline. The second subscriber's modem interprets the analogue signals onthe analogue subscriber line by demodulating the QAM signals produced bythe first subscriber's modem. The same way of data communication iscarried out in the reverse direction.

Increasingly more subscribers are connected to the telephone networkthrough a digital subscriber interface, such as ISDN. Thus, many dataconnections are established between a first subscriber having ananalogue network interface and a second subscriber having a digitalnetwork interface. In many cases, the second subscriber will be aninternet service provider. In order to optimise data transmission oversuch heterogeneous communication channels, various proposals have beenmade in the recent past. One such proposal is known from U.S. Pat. No.5,801,695.

The proposal is based on the idea that the transmission rate in aheterogeneous communication channel from the digital subscriber to theanalogue subscriber may be raised by using a PCM coding techniqueinstead of the former QAM modulation techniques. The PCM codingtechnique uses a plurality of signal levels for encoding data symbols(each data symbol comprising multiple bits). These signal levels areagain recognised by the receiving modem which is then able to decode thedata symbol encoded into the signal levels.

Further, ITU has promulgated a new recommendation V.90 in Sep. 1998. Thenew recommendation also relies on a PCM coding technique for thetransmission of data from the digital subscriber to the analoguesubscriber. Draft recommendation V.90 in terms of its PCM coding schemedepends on ITU-T recommendation G.711 describing Pulse Code Modulation(PCM) of Voice Frequencies which is generally applied in telephonenetworks throughout the world when converting analogue signal amplitudevalues into numeric representations thereof, and vice versa. G.711recommends two PCM coding schemes generally known as μ-law, which isapplied in North American telephone networks, and A-law, which isapplied inmost other telephone networks. Both coding schemes have incommon that they have a logarithmic coding characteristic, i.e. thelower the signal amplitude value to be encoded, the more fine-grain theavailable PCM codes. Such logarithmic coding characteristic has beenfound to be particularly advantageous for encoding analogue voicesignals at minimum distortion.

Recommendation G.711 makes available 256 PCM codes (or U-codes as calledin V.90) which are grouped into eight positive and eight negativesegments (or U-chords as called in V.90). Each PCM code is encoded usingeight bits. Due to power restrictions on the analogue telephone line anddue to line impairments, the analogue modem (according to theterminology used in the draft to V.90) receiving analogue amplitudevalues is unable to discriminate between all 256 available PCM codes.Therefore, a reduced set of PCM codes is determined for encoding datasymbols during set-up of a data communication channel under real worldconditions. This accordingly lowers the data transmission rate down fromthe maximum theoretical possible value of 64 kbit/s such that it is notabove 56 kbit/s.

ITU-T Recommendation V.90 assumes an environment where one subscriberterminal of a connection is connected to the telephone network through adigital line and the other subscriber terminal of the connection isconnected to the telephone network through an analogue line. However, inmany instances, both subscriber terminals of a connection are connectedto the telephone network through a digital connection. This situationwould allow to establish an all-digital channel between the twosubscriber terminals at a data rate of 64 kbit/s, which is the standarddata rate in telephone networks.

U.S. Pat. No. 5,515,398 discloses modem line probing signal techniques.These probing techniques relate to former analogue line modems definedfor example in ITU-T V.34.

It is an object of the invention to provide a line probing scheme inorder to detect an all-digital connection path between subscriberterminals of a telephone network.

A method of determining properties of a signal transmission channel in atelephone network that connects a first subscriber end point to a secondsubscriber endpoint by a signal transmission channel having a digitalchannel portion is disclosed. The method can include sending a digitalprobing signal from a first subscriber terminal to a second subscriberterminal. The method can also include receiving, at the secondsubscriber terminal, a received signal resulting from having transmittedthe digital probing signal through the signal transmission channel andcomparing the received signal with the digital probing signal todistinguish between possible channel configurations of the signaltransmission channel. The method can also include transmitting aresponse signal from the second subscriber terminal to the firstsubscriber terminal.

A first embodiment of the invention pertains to a method of determiningproperties of a signal transmission channel between a first subscriberend point and a second subscriber end point of a telephone networkhaving a plurality of subscribers. A first subscriber terminal isconnected to said first subscriber end point and a second subscriberterminal is connected to said second subscriber end point. Saidtelephone network upon request of a subscriber establishes a signaltransmission channel between said first subscriber and said secondsubscriber. Said first subscriber end point is connected to thetelephone network by a digital channel portion. In a first step, saidfirst subscriber terminal sends to said second subscriber terminal adigital probing signal comprising a sequence of frames, each framecomprising a sequence of digital symbols, each symbol having a pluralityof bits. The digital values of all symbols over all frames are equalexcept for one bit position of each symbol, the value of which changeswith every other frame. Said second subscriber terminal then receives asignal which is the result of said digital probing signal having beentransmitted through said signal transmission channel. Said secondsubscriber terminal evaluates said received signal by comparing saidreceived signal with said digital probing signal. Eventually, saidsecond subscriber terminal transmitting a response signal to said firstsubscriber terminal, said response signal carrying information about thecomparison result.

A second embodiment of the invention also concerns a method ofdetermining properties of a signal transmission channel between a firstsubscriber end point and a second subscriber end point of a telephonenetwork having a plurality of subscribers. This method alternativelyprovides that said first subscriber terminal sends to said secondsubscriber terminal a digital probing signal comprising a sequence offrames, each frame comprising a sequence of digital symbols, each symbolhaving a plurality of bits, wherein the digital values of all symbolsare equal except for at least one pulse symbol of each frame having asignificantly different digital value compared to the remaining equalvalues.

The line probing schemes proposed by the invention allow both to findout whether an all-digital transmission channel is present between saidfirst subscriber end point and said second subscriber end point andfurther to find out the transmission properties of the all-digitaltransmission channel. Since the transmission channels of most telephonenetworks are primarily intended for voice signal transmission, somenetworks impose digital signal impairments upon the digital signalscarried through the network's channels. Such digital impairments includedigital padding (digital signal attenuation), robbed bit signalling,ADPCM (Advanced Differential Pulse Code Modulation) coding and voicecompression algorithms. The latter impairments allow to reduce the bitrate of 64 kbit/s generally reserved for a full channel to a lower ratewithout much sacrifice to the quality of voice signal transmission, thusmaking available bandwidth for other purposes. The methods of theinvention are capable of discriminating whether an all-digital channelis present and whether or not the all-digital channel has digitalimpairments. The methods of the invention are even capable ofdiscriminating what kind of digital impairment is present in anall-digital transmission channel. Knowing the kind of digital impairmentallows the conclusion as to whether a transmission scheme between saidfirst subscriber terminal and said second subscriber terminal ispossible according to V.90 or another lower rate scheme such as V.34.

In the first embodiment, it is preferred that said one bit position isthe most significant bit position. This way, the absolute digital valuedifference from one frame to another is as large as possible. It is evenfurther preferred that said one bit position is the position of the signbit. This way of line probing ensures that no direct current is producedin an analogue channel portion which may be present in the transmissionchannel to be probed.

In the second embodiment, it is preferred that one bit position of saidat least one pulse symbol changes value with every other frame. Thus,frames can be identified as such more easily by the second subscriberterminal. In an even more preferred embodiment, said one bit position isthe position of the sign bit. This ensures that no direct current isproduced in an analogue channel portion which may be present in thetransmission channel to be probed.

In the second embodiment, it is preferred that the number of equalsymbols per frame is significantly higher than the number of pulsesymbols. This allows said second subscriber terminal to clearly identifya pulse symbol as such. Preferably, there is one pulse symbol per frame.

Alternatively, there may be two pulse symbols per frame. It is mostpreferred that the total number of symbols per frame is 80.

Further advantages, features and areas of using the invention areexplained in the following description of a preferred embodiment of theinvention which is to be read in conjunction with the attached drawings.In the drawings:

FIG. 1 a shows the configuration of an all-digital signal transmissionpath in the presence of digital impairment;

FIG. 1 b shows the configuration of a signal transmission path having ananalogue portion;

FIG. 2 is a signal diagram of a probing signal and a received signalaccording to a first embodiment of the invention;

FIG. 3 depicts a digital symbol sequence of a probing signal and variousreceived signals according to a second embodiment of the invention.

FIG. 1 a illustrates an all-digital signal transmission path between afirst subscriber terminal 1 and a second subscriber terminal 8. Thefirst subscriber terminal 1 (a digital modem) is connected through adigital line portion 2 to a local digital switch 3. The local switch 3is connected to a digital transmission network 4 which forwards digitalsignals between subscribers of the transmission network. On the otherend of the all-digital signal path, the second subscriber terminal 8 isconnected through a digital line portion 7 to a local digital switch 6.The local switch 6 is connected to the transmission network 4 through adigital impairment device 5. FIG. 1 a shows an exemplary position of thedigital impairment device within the transmission path. The digitalimpairment device may as well be part of any of the digital switches 3and 6 or may be part of the transmission network 4 or of thetransmission path 7.

Digital impairments include digital padding (digital signalattenuation), robbed bit signalling (RBS), and ADPCM (AdvancedDifferential Pulse Code Modulation) coding or other voice compressionalgorithms which may be imposed upon the signals passing through theimpairment device 5. Digital impairment devices are present in manyexisting transmission networks and have to be accounted for when tryingto establish a connection between subscriber terminals of the network atthe highest bit rate possible.

FIG. 1 b shows a similar configuration as FIG. 1 a except that thesecond subscriber terminal 8′ is connected to the transmission networkthrough an analogue line portion 7′. The transmission path of FIG. 1 aconsequently includes a hybrid device 9′ which is connected to theanalogue line portion 7′ and performs a four-wire to two-wireconversion. Additionally, the hybrid device 9′ performs, on thefour-wire side, a digital-to-analogue and an analogue-to-digital signalconversion so as to be connected to a digital switch 6′. The remainingstructure of FIG. 1 b corresponds to the one shown in FIG. 1 a. Thus,the description of the remaining elements may be referred to by similarreference numerals.

Both FIG. 1 a and FIG. 1 b illustrate exemplary structures oftransmission paths that may be encountered when trying to establish aconnection between two subscribers of a transmission network wherein atleast one of the two subscribers is connected to the network through adigital line portion such as ISDN. Depending on the structureencountered on the transmission path between the subscribers, they mayagree upon a certain transmission scheme allowing a bit rate as high aspossible for the encountered structure. Known transmission schemes areITU-T V.34 using quadrature amplitude modulation on analoguetransmission paths and ITU-T V.90 using pulse amplitude modulation ontransmission paths having both analogue and digital line portions.Further, pulse amplitude modulation according to ITU-T V.90 can also beused as a transmission scheme on all-digital transmission paths.

FIG. 2 is a diagram of a probing signal of the first embodiment of theinvention. The probing signal is transmitted by the first subscriberterminal 1. FIG. 2 also shows a signal received by the second subscriberterminal 8 in the presence of a digital impairment device 5 introducingADPCM to the signal transmission path between the first subscriber andthe second subscriber. Terminal 1 sends 80 digital symbols of equalvalue in a first frame and then sends 80 digital symbols of the sameabsolute value, however, being negative in sign. The probing signalconsists of a plurality of frame pairs as illustrated in FIG. 2subsequently transmitted by the first terminal 1.

In the presence of ADPCM in the transmission path, the received signaldoes not precisely follow the large signal swings from one frame toanother. The second terminal 8 may interpret this as an all-digitaltransmission path which is not transparent due to ADPCM. Such aconnection is not capable of carrying an ITU-T V.90 transmission scheme.

FIG. 3 shows a digital symbol sequence of a probing signal (sequence a)transmitted by the first subscriber terminal 1 (Modem 1) according to asecond embodiment of the invention and various cases of received signals(sequences b through f). FIG. 3 shows a frame structure of . . .

Sequence (b) of FIG. 2 shows the signal received by subscriber terminal8 (Modem 2) in the case of an all-digital, fully transparent connection.Thus, the frame sent by modem 1 is received by modem 2 with identicalsymbols, merely displaced in time. This case allows to establish a PCMtransmission scheme between modem 1 and modem 2. Sequences (b) though(f) show received signals in the presence . . .

Sequence (d; shows a received signal in the presence of digitalimpairment in the form of robbed bit signalling (RBS). RBS is applied toa least significant bit of every sixth symbol. Thus, the received signaldiffers from the original probing sequence every sixth symbol. Sequence(e) shows a received signal in the presence of both digital padding andRBS. Thus, the effects of both impairments appear as a superimposedeffect on the received signal.

Finally, sequence (f) shows a received signal under the influence ofADPCM or another voice compression algorithm. The ADPCM coder cannotfollow the high pulse symbol 4Ch of the probing sequence interspersed inthe zero symbols 00h. Thus, the pulse symbol of modem 1 is received witha much wider pulse width and less high amplitude in modem 2. This is aclear indication of ADPCM.

The invention generally also relates to a modem connected via a digitalinterface to a switched public telephone network. The modem communicateswith a second modem also connected via a digital interface to the sameswitched public telephone network. Thus, there may exist the possibilityto set up a connection between both modems with a transmission rate of64 kbit/s on the basis of coding voice signals with pulse codemodulation according to ITU-T recommendation G.711. The public telephonenetwork may incorporate voice compression devices (ADPCM G.726, G.723etc.), digital pads (digital attenuators), robbed bit signalling andecho cancelling devices.

Under such circumstances, digital encoding schemes like pulse amplitudemodulation according to ITU-T V. 90 instead of known analogue schemeslike ITU-T V.34 may be utilised to transfer data. In order to apply sucha digital coding scheme, it needs to be assured that an all-digitalchannel has been established between the modems. Assuring this may becarried out by an appropriate probing signal sent through thetransmission channel. Known probing techniques have proven that theycannot discriminate all possible channel configurations.

The probing sequence of the invention uses large amplitude changes in asymbol sequence (each symbol having a duration of 125 μs). The meaningof amplitude relates to the definition of ITU-T recommendation G.711.After that single amplitude change, the signal may return to theprevious value or continue with the new amplitude value for a number ofsymbols. The number of symbols is selected to be larger than anyexpected impulse response of a digital impairment of the channel. Theamplitude value change mast be large enough to produce a sufficientresult in the presence of digital pads with or without the presence ofRBS.

The receiving modem will evaluate the received symbols and search foramplitude changes. If these changes occur only for one symbol per frameand the following symbols either return to the previous value or remainat the new value, the connection is detected as capable of carrying, aV.90 transmission scheme. If, however, the symbols after an amplitudechange do not remain at the new value or do not return to the valuebefore the change (in other words there is an impulse response overtime), it is determined that a connection according to ITU-T V.90 is notpossible. Typical impairments having an impulse response are voicecompression algorithms and ADPCM, which may also be regarded as acompression algorithm. Whereas ADPCM has a characteristic impulseresponse to a change in amplitude, it depends on the design of a voicecompression algorithm how large amplitude swings are processed and codedinto the output signal of the voice compression coder.

Robbed bit signalling changes the least significant bit (LSB) in somesymbols but leaves the remaining seven bits unchanged. A singleamplitude change will therefore only be affected in the LSB, theremaining seven bits, however, will not change. Digital pads useconversion functions which defines an output value to a PCM input valuethus providing digital attenuation. This function will only change theabsolut value of the amplitude but it will not affect the behaviour ofthe signal over time.

In an implementation, modem 1 will generate a pattern as described inconjunction with FIG. 3 and sends the pattern through the transmissionchannel to modem 2. Modem 2 will receive a pattern which differs fromthe transmit pattern due to network impairments. Modem 2 will evaluatethe pattern in the following way: It will first logical AND the patternwith FEh in order to ignore changes in the LSB. Next it will comparethis new value with the previous one. If they are identical, a counteris incremented. If they are different, then the current count value iscompared to the expected value and if they differ, an error counter isincremented. Then the counter is reset to zero and the new value istransferred to the old value register. When all symbols have beenevaluated, the value of the error counter is compared to a fixedthreshold value. If the error counter value is below the threshold, thenit is determined that the connection is not capable of carrying an ITU-TV.90 type transmission scheme.

The appended program codes show how line probing signals according tothe invention may be produced. The programs are based on a pseudo code.The program of appendix A corresponds to the embodiment of FIG. 2, andthe program of appendix B corresponds to the embodiment of FIG. 3. By nomeans are these programs a limitation of the invention.

APPENDIX A Example of program code for alternating pattern:Transmit_Pattern: Loop_COUNT = 20 High_Code = 4CH Low_Code = CCHTotal_count = 100 For (i; I=0; I=Total_count) { For (j;J=1;j=Loop_Count) SendPCMvalue (Low_code); For (j;J=i;j=Loop_Count)Ser.dPCMvalue(High_Code); } Receiver: Loop_Count = 20 Total_count =100 * 2 * Loop_Count Old = 0; Error = 0; Count = 0; For(i;i=0;i=Total_count) { a = (GetnewPCMvalue( ) && 11111110B) ;Masked LSBfor RRS impact If (Old < > a) count++ ELSE IF (COUNT < > LOOP_COUNT−1)Error++ count = 0 Old = a; } If (Error > 1) Return (False) Return (True)

APPENDIX B Example code for single value pattern detection:Transmit_Pattern: Loop_COUNT = 20 High_Code = 4CH Low_Code = 00HTotal_count = 100 For (i;I=0;I=Total_count) { For (j;J=1;j=Loop_Count−1) SendPCMvalue (Low_code); SendPCMvalue High_Code); For(j;J=1; j=Loop_Count−1 SendPCMvalue (Low_code); SendPCMvalue(High_CodeEXOR 80H): } Receiver: Loop_Count = 20 Total_count = 100 * (LoopCount)Old = 0; Error = 0; Count = 0; For (i;i=0;i=Total_count) { a =(GetnewPCMvalue( ) && 11111110B) ;Masked LSB for RBS impact If (Old < >a) count+ + ELSE { IF (COUNT < > LOOP_COUNT−2) IF (COUNT < > 0) Error++Count = 0 Old = a; } If (Error > 1) Return (False) Return (True)

1. In a telephone network connecting a first subscriber end point to asecond subscriber end point by a signal transmission channel having adigital channel portion, a method of determining properties of saidsignal transmission channel, said method comprising: sending a digitalprobing signal from a first subscriber terminal connected to said firstsubscriber end point to a second subscriber terminal, connected to saidsecond subscriber end point, said digital probing signal having asequence of probing frames, each probing frame having at least two frameportions, each frame portion having the same preset number of digitalsymbols, each digital symbol having one sign bit and one data bit,wherein absolute digital values of all symbols in the frame portions areequal, and wherein a value of the sign bit changes with every adjacentframe portion, receiving, at said second subscriber terminal, a receivedsignal resulting from having transmitted said digital probing signalthrough said signal transmission channel; comparing said received signalwith said digital probing signal to distinguish between possible channelconfigurations of said signal transmission channel; and transmitting aresponse signal from said second subscriber terminal to said firstsubscriber terminal, said response signal carrying informationindicative of a result of comparing said received signal with saiddigital probing signal.
 2. The method according to claim 1, whereinsending a digital probing signal comprises setting all data bits of eachsymbol of a probing frame to have the same logical value.
 3. The methodaccording to claim 1, wherein sending a digital probing signal comprisessetting the total number of symbols of a probing frame to be greaterthan the number of symbols in an impulse response of a digitalimpairment of the signal transmission channel.
 4. The method accordingto claim 3, wherein setting the total number of symbols of a probingframe further comprises selecting the total number of symbols perprobing frame to be
 80. 5. A subscriber terminal connected to asubscriber end point of a telephone network having a plurality ofsubscribers, said subscriber terminal comprising: a connection betweensaid subscriber terminal and a subscriber end point, said subscriber endpoint being connected to the telephone network by a digital channelportion, and a probing signal transmitter for sending, to a secondsubscriber terminal to which a signal transmission channel has beenestablished, a digital probing signal having a sequence of probingframes, each probing frame having at least two frame portions, eachframe portion having the same preset number of digital symbols, eachdigital symbol having one sign bit and one data bit, wherein absolutedigital values of all symbols in the frame portions are equal, andwherein the value of the sign bit changes with every adjacent frameportion.
 6. The subscriber terminal of claim 5, wherein one bit positionof said digital symbol changes value with every other frame.
 7. Thesubscriber terminal of claim 6, wherein said one bit position is theposition of the sign bit.
 8. The subscriber terminal of claim 5, whereinthe number of equal symbols per frame is higher than the number ofdigital symbols.
 9. The subscriber terminal of claim 5, wherein there isone digital symbol per frame.
 10. The subscriber terminal of claim 5,wherein there are two digital symbols per frame.
 11. The subscriberterminal of claim 5, wherein the total number of symbols per frame is80.
 12. A telephone network comprising: a connection between asubscriber end point of said telephone network and a first subscriberterminal, said subscriber end point being connected to the telephonenetwork by a digital channel portion, and a probing signal transmitterfor sending, to a second subscriber terminal to which a signaltransmission channel has been established, a digital probing signalhaving a sequence of frames, each frame having a sequence of digitalsymbols, the sequence including the same preset number of digitalsymbols, each symbol having one sign bit and one data bit, whereindigital values of all symbols over all frames are equal except for onebit position of each symbol, the value of which changes with every otherframe.
 13. A telephone network comprising: a connection between asubscriber end point of said telephone network and a first subscriberterminal, said subscriber end point being connected to the telephonenetwork by a digital channel portion, a probing signal transmitter forsending, to said second subscriber terminal, a digital probing signalhaving a sequence of frames, each frame having a sequence of digitalsymbols, the sequence including the same preset number of digitalsymbols, each symbol having a one sign bit and one data bit, whereindigital values of all symbols are equal except for at least one symbolof each frame having a different digital value compared to the remainingequal values.